The invention relates to a method for measuring and/or regulating the quantity of heat which is present in a fuel gas and is supplied to a gas-consuming device, in particular a natural-gas-consuming device, the calorific value of the fuel gas being used as an input variable.
In known measurement methods of this nature, the amount of gas supplied and the heat properties of the fuel gas are often determined. The conditions, in particular the temperature and pressure, under which these values are determined generally differ for each measurement variable.
For example, the amount of gas supplied is often measured under operating conditions, while the heat properties are often determined under standardised conditions, such as for example the normal conditions 0xc2x0 C. and 1.01325 bar(a). To determine the quantity of heat, it is important for uniform temperature and pressure conditions to be observed both for the amount of gas supplied and the thermal properties. In practice, the amount of gas supplied is to this end generally converted to standardised conditions, for example the normal conditions 0xc2x0 C. and 1.01325 bar(a). This conversion is known as xe2x80x98volume conversionxe2x80x99.
The quantity of heat supplied to the gas-consuming devices can be determined by means of direct and indirect methods. In indirect methods, the composition of a natural gas is determined by means of gas chromatography, for example. Then, on the basis of this gas composition, the parameters for the volume conversion are calculated and the calorific value of the fuel gas is determined using the calorific values of the pure substances. Although these methods provide very accurate results, they have the drawback of being technically complex and therefore expensive. As a result, it is impossible to use these methods in private households, for example. In contrast to indirect methods, in direct methods the calorific value is determined directly. Commercially available calorific-value meters indicate the calorific value, generally under standardised conditions, such as for example normal conditions (0xc2x0 C. and 1.01325 bar(a)). Usually, the volume conversion which is required to determine the flow of energy is derived from density measurements under standardised conditions, such as for example normal conditions (0xc2x0 C. and 1.01325 bar(a)), and the conditions of the volumetric flow measurement. However, volume conversion based on density measurements are technically complex. Moreover, density cells of this nature have to be calibrated at regular intervals.
The object of the invention is to reduce the effort involved in the direct measurement and/or regulation of the quantity of heat supplied to consumption devices and, in particular, to provide a reliable and accurate measurement method.
According to the invention, this object is achieved by the fact that, in the method referred to in the introduction, the fuel gas or a part-stream of the fuel gas is guided through a volumetric meter or a mass flow meter, and the volumetric flow rate or the mass flow rate is measured, the speed of sound of the gas is determined under first reference conditions, one of the measurement variables dielectric constant, speed of sound under second reference conditions, carbon dioxide content of the fuel gas, nitrogen content of the fuel gas or density under standardised conditions, for example normal conditions (0xc2x0 C. and 1.01325 bar(a))is measured; and the quantity of heat supplied is derived from these parameters, together with the calorific value of the fuel gas, as a measurement variable or control variable.
The advantage of this method lies in the fact that there is no need to carry out any density measurements under any conditions apart from standardised conditions. In most embodiments, there is no need to carry out any density measurement at all.
Particularly accurate results can be achieved for fuel gases whose calorific value at normal conditions is from 20 to 48 Mj/m3, whose relative density compared with dry air is from 0.55 to 0.9, whose proportion of carbon dioxide is less than or equal to 0.3 and whose proportion of hydrogen and carbon monoxide is less than 0.1 and 0.03 respectively. Particularly suitable measurement conditions are temperatures in the range from 225 to 350 K and pressures of less than or equal to 6 MPa.
Operating conditions are the actual conditions in the installation, for example a gas conduit, containing the gas of which the quantity of heat is measured or controlled. Reference conditions can be chosen freely within the specified ranges, preferably corresponding to conditions at which the relevant parameters are known from one or more reference gases. By standardised conditions are denoted conditions that are more generally used in the relevant technical field like normal conditions (0xc2x0 C. and 1,01325 bar(a)) and standard conditions (15xc2x0 C. and 1,01325 bar(a)).
The first reference conditions set are preferably normal conditions or a pressure in the range from 1 to 10 bar, more preferably from 3 to 7 bar. Although the temperature is not very critical and can be selected within a wide range, for technical reasons the temperature is above 225 K, for example from 270 K to 295 K. For the second reference conditions, a pressure of above 30 bar is preferably set. Although the temperature is not very critical and can be selected within a wide range, for technical reasons the temperature is from 225 K to 350 K.
The operating conditions are most preferable for this parameter.
The speed of sound at the reference conditions mentioned, including operating conditions, can be determined in a separate measuring unit, for example via the resonant frequency of vortex pipes or of hollow bodies or a distance travelled-time measurement, e.g. in ultrasonic flow meters.
In practice, there are various proven measurement methods available for measuring the volumetric flow rate, for example turbine flow meters or ultrasonic flow meters.
The dielectric constant can be measured inexpensively and with high accuracy even under operating conditions. The proportion of carbon dioxide is simple to determine under all conditions mentioned using known measuring instruments, e.g. by measurement of the light absorption in the infrared region.
The use of the density under normal conditions is advantageous in particular when, in the arrangement, a volume conversion is at the same time being carried out on the basis of density measurements. The use of a speed of sound measurement instead of the density measurement under operating conditions offers the advantage that the most critical component is exchanged, while there is no need to spend money on measuring other variables.
Consequently, the three measurements which are required can each be carried out reliably, accurately and without a high level of technical effort, so that linking the measured values provides suitable results for measuring and/or regulating the quantity of heat supplied to gas-consuming devices.
To establish the reference conditions, the parameters temperature and pressure are required. These can be additionally measured in step b). If a lower measurement accuracy is permissible, the values estimated from practice can also be used for these parameters.
If the calorific value does not change, or changes only slightly, such as for example in the case of a gas emanating from the same source, it is sufficient to introduce a fixed value for the calorific value into the calculation of the quantity of heat. In the event of substantial fluctuations in the calorific value of a gas flow, as may occur, for example, in collection networks, it is recommended that the calorific value be determined at regular intervals or continuously. To this end, the calorific value can be recorded inexpensively and with a high level of accuracy using various proven measurement methods, such as for example controlled catalytic oxidation of the gas to be tested.
In total, there are five variations on the method according to the invention for measuring the quantity of heat supplied of fuel gas.
In all the variants, the speed of sound is recorded under first reference conditions and the calorific value is determined or input as a constant.
In addition, in the first variant the speed of sound is also recorded under second reference conditions. Recording two speeds of sound has the advantage that the second measurement can be carried out in the same measuring device. The pressure in the apparatus can be varied by compressing the measurement volume or allowing it to expand. During the compression or expansion, the temperature of the fuel gas also changes, making it easier to set altered reference conditions. If desired, the measuring device for determining the speed of sound may also be equipped with means for varying the temperature setting. In addition, in the second variant, the dielectric constant is also measured, preferably at a pressure of at least 10 bar, e.g. under second reference conditions, e.g. operating conditions, in order to achieve a high level of accuracy.
In the third variant, the carbon dioxide content of the fuel gas is also determined.
The determination of the dielectric constant and of the carbon dioxide content may be carried out in the same measurement environment as that in which the speed of sound is determined. This allows the measuring device to be extremely compact.
In contrast to the third variant, in the fourth variant the nitrogen content is also determined, instead of the carbon dioxide content.
In the fifth variant, the density is additionally recorded under normal conditions. This variant offers advantages especially when refitting an existing volume-conversion installation which is based on density measurement, since in this specific situation it is sufficient to use only one speed of sound recording instead of the density measurement under operating conditions. As a result, on the one hand, the volume conversion is considerably simplified and is made easier to control, while, on the other hand, the existing instrumentation is used to its maximum possible effect.
Advantageously, the speed of sound is recorded under second reference conditions and the dielectric constant or the carbon dioxide content is recorded under the same reference conditions, preferably under operating conditions, in a common measurement environment. In this way, only one temperature and pressure measurement, and consequently only one thermostat, are required in order to produce or maintain the reference conditions. Moreover, uniform reference conditions for the various measurements increase the accuracy with which the quantity of heat supplied can be determined.
Recording at least one speed of sound in addition to the calorific value offers the further advantage that it is possible to dispense with determining the density of the fuel gas under operating conditions. Apparatus for measuring the density at operating conditions are expensive and complex. Preferably, in the method according to the invention, in particular if the carbon dioxide content is recorded as the third measurement variable, no additional density measurement is carried out.
To find a suitable correlation between the set of parameters applied in the method according to the invention, for example between the speed of sound under first reference conditions, the speed of sound under second reference conditions, the calorific value and the density under operating conditions and standardised conditions, it is advantageous to precede the respective steps b) and c) at least once by a plurality of measurement cycles in which step b) is carried out using a plurality of reference gases of known calorific value. The parameters required for the various variants of the method are then measured on the reference gas. In these reference cycles, a number corresponding to the number of measurement cycles of reference signal patterns determined from the ratio of the various signals measured are stored with assignment to the known densities at operating and standardised conditions. The signal pattern from a future measurement cycle on fuel gas of the unknown density at operating or standardised conditions is compared with the reference signal patterns so as to assign a particular density at operating or standard conditions.
To increase the reference accuracy, many reference cycles in which the various parameters are varied in succession over the expected measurement range should be carried out. An unambiguous and accurate assignment of a particular density at operating or standardised conditions to a signal pattern of a fuel gas determined in a measurement cycle is achieved by interpolation of the various reference signal patterns.
A significant advantage is that the correlation between densities at operating and/or standardised conditions and measured parameters only has to be found once for a specific application by means of any desired number of reference cycles. The one-off effort is comparatively low. The reference conditions should here be selected so as to correspond as closely as possible to the measurement conditions expected later. Thus, for all parameters only the measurement ranges which actually come into question should be determined with sufficient accuracy as reference signal patterns.
If the composition of the fuel gas may display greater variations, it is generally necessary to determine more reference signal patterns.
Large quantities of data are already available in relation to the dependency of the gas composition on the speed of sound and the density. Using this available data therefore makes it possible to calculate the calorific value, the speeds of sound and the density under operating conditions and standardised conditions as a function of the gas composition in the relevant area. It is thus possible to replace expensive measurements with calculations.
A preferred embodiment of the invention is characterised in that the respective proportion of a specified number of alkanes, including methane, is determined by determining the proportion of the individual alkanes, excluding methane, with the aid in each case of an associated function dependent on a selected physical property, preferably the molar calorific value, of the sum of the specified alkanes and in that the proportion of methane is determined from the difference between the proportion of the sum of the specified alkanes and the sum of the proportions of the alkanes determined by means of the functions.
As specified alkanes, all alkanes which are actually present in the fuel gas should, if possible, be selected and specified.
It has been found that the proportions of the alkanes in natural fuel gases are always in a particular ratio to one another which depends only on a physical property, e.g. the molar calorific value, of the sum of the specified alkanes. This is obviously attributable to the fact that natural gases in the form in which they occur have always gone through an equilibrium phase in which their gaseous and liquid phases have been in equilibrium with one another. However, the method is not restricted to natural fuel gases, either with or without addition of coal gas. For synthetic gases containing added substances or for gas mixtures having many components, the uncertainty in the determination of the gas composition is merely somewhat greater.
The molar calorific value of the sum of the specified alkanes can in turn be determined, for example, with the aid of reference signal cycles. Since the composition of the reference gases is known, their molar calorific value of the sum of the specified alkanes is also known. Consequently, from the ratio of the signals measured on the reference gases, a number corresponding to the number of reference measurement cycles of reference signal patterns can be stored with assignment to the known molar calorific values of the sum of the specified alkanes. In a future measurement cycle, the molar calorific value of the sum of the specified alkanes in the fuel gas can be determined merely by comparison of the signals measured with the stored reference signal patterns.
As functions for determining the proportions of the individual alkanes with the exception of methane, use can advantageously be made of polynomials, preferably of second order.
In a preferred illustrative embodiment, the proportions of methane, ethane, propane, isobutane, n-butane, isopentane, n-pentane, hexane, heptane and octane are determined with the aid of the functions. It has been found that the proportion of all further hydrocarbons can be ignored, particularly in the case of natural fuel gas. The relationship between the molar calorific value and the sum of the specified alkanes is in this case, e.g.:
xe2x80x83XC2H6=[xcex11(HCHxe2x88x92HCH4)+xcex21(HCHxe2x88x92HCH4)2]XCHxe2x80x83xe2x80x83(1.1)
XC3H8[xcex12(HCHxe2x88x92HCH4)+xcex22(HCHxe2x88x92HCH4)2]XCHxe2x80x83xe2x80x83(1.2)
Xi-C4H10=[xcex13(HCHxe2x88x92HCH4)+xcex23(HCHxe2x88x92HCH4)2]XCHxe2x80x83xe2x80x83(1.3)
Xn-C4H10=[xcex14(HCHxe2x88x92HCH4)+xcex24(HCHxe2x88x92HCH4)2]XCHxe2x80x83xe2x80x83(1.4)
Xi-C5H12=[xcex15(HCHxe2x88x92HCH4)+xcex25(HCHxe2x88x92HCH4)2]XCHxe2x80x83xe2x80x83(1.5)
Xn-C5H12=[xcex16(HCHxe2x88x92HCH4)+xcex26(HCHxe2x88x92HCH4)2]XCHxe2x80x83xe2x80x83(1.6)
Xn-C6H14=[xcex17(HCHxe2x88x92HCH4)+xcex27(HCHxe2x88x92HCH4)2]XCHxe2x80x83xe2x80x83(1.7)
Xn-C7H16=[xcex18(HCHxe2x88x92HCH4)+xcex28(HCHxe2x88x92HCH4)2]XCHxe2x80x83xe2x80x83(1.8)
Xn-C8H18=[xcex19(HCHxe2x88x92HCH4)+xcex29(HCHxe2x88x92HCH4)2]XCHxe2x80x83xe2x80x83(1.9)
Here, xcex1i and xcex2i are constants and HCH4 is the molar calorific value of methane. The variable HCH is the molar calorific value of the sum of the specified alkanes (HCH=xcexa3XCH,iHCH,i). The proportion of methane is in this case determined as follows:
XCH4=XCHxe2x88x92(XC2H6+XC3H8+Xi-C4H10+Xn-C4H10+Xi-C5H12+Xn-C5H12+Xn-C6H14+Xn-C7H16+Xn-C8H18)xe2x80x83xe2x80x83(2) 
An embodiment of the invention is characterised in that the steps b) and c) are preceded by a plurality of measurement cycles in which step b) is carried out using a plurality of reference gases whose composition and whose selected physical property of the sum of the specified alkanes are known, in that the constants, e.g. coefficients, of the functions describing the proportion of the alkanes excluding methane are determined from the signals measured on the reference gases, in that the constants of the functions are stored with assignment to the respective alkanes and in that the proportion of the alkanes excluding methane is determined from a future measurement cycle on fuel gas of unknown composition with the aid of the functions.
In this way, the constants xcex1i and xcex2i can be found with the aid of only two reference cycles for all natural gases. To increase the measurement accuracy, any number of reference cycles can be carried out. Even in the case of a large number of reference cycles, the effort remains comparatively low since the constants xcex1i and xcex2i only have to be determined once.
The proportion of the sum of the alkanes in the fuel gas can be determined with the assumption that the fuel gas consists only of a specified number of alkanes, nitrogen and carbon dioxide. The associated equations therefore have the form:
XCH=1xe2x88x92XN2xe2x88x92XCO2xe2x80x83xe2x80x83(3)
Here, XN2 and XCO2 are the proportions of nitrogen and carbon dioxide, respectively.
To increase the measurement accuracy, it can alternatively be assumed that the fuel gas additionally contains hydrogen and/or carbon monoxide. If both the proportion of hydrogen and the proportion of carbon monoxide are taken into account, equation (3) becomes:
XCH=1xe2x88x92XN2xe2x88x92XCO2xe2x88x92XH2xe2x88x92XCOxe2x80x83xe2x80x83(3xe2x80x2)
A preferred illustrative embodiment is characterised in that the speeds of sound under two different conditions and the calorific value can be derived from the gas composition. These parameters can be calculated in a simple way from the composition of the gas by means of an equation of state such as the AGA-8 equation frequently used for gas.
Wi,calc=f(pi, Ti, XCH4, . . . Xn-C8H18, XCO2, XN2)xe2x80x83xe2x80x83(4)
An example of an embodiment of the invention is characterised in that values for the speed of sound under first and second reference conditions and the calorific value are derived from the composition of the fuel gas, in that the difference between the derived and the measured value of the speeds of sound and the calorific value is formed, in that, if the difference exceeds a specified threshold value, the proportion of at least one of the components of the fuel gas to be determined is altered, in that the composition is recalculated on the basis of the altered value or values, the values of the selected parameters are recalculated and the difference between these and the measured values is determined and in that the latter two steps are repeated until the difference lies below the threshold value.
It has been found that the gas composition can be determined particularly quickly in this way. If all 14 equations given above (1.1 to 4) are to be taken into account, these contain up to 14 unknowns, namely XCH, XCH4, XC2H6, XC3H8, . . . , Xn-C8H19, W1,calc, W2,calc, HS,n,calc, xcfx81m,n,calc. To solve these 14 equations, one or more values, e.g. the two speeds of sound mentioned in combination with the calorific value, can be derived and compared with the corresponding measured value.
The advantage of the method according to the invention is that not only is it possible to determine the quantity of heat supplied to consumption devices, but also other important properties of the gas such as the compressibility factor, the density, the speed of sound, the enthalpy, the methane number or the Wobbe index of the fuel gas can be calculated from the composition.
The physical parameters determined using the method of the invention are essentially as good as those determined by means of gas chromatography. According to the invention, only three measured parameters, e.g. two speeds of sound and the calorific value, are sufficient to carry out many process engineering calculations. Firstly, changes of state in gas reservoirs or storage volumes can be determined. In addition, the relevant gas transport data,.e.g. temperature or pressure drop, can be determined. For vehicles powered by natural gas, the required design of gas filling stations can be calculated.
Fill level measurements can be checked and designed using the method of the invention.
In connection with heat exchangers too, the invention is of great advantage. The design of heat exchangers can be calculated using the method of the invention. Performance measurements on heat exchangers can be evaluated using the method. Finally, compressor characteristics and compressor performances can be determined using the method of the invention.
In all the above mentioned applications, expensive gas-chromatographic analyses have previously been necessary.
The methane number, too, can be determined using the method of the invention. If the gas property data which were originally measured for charging purposes are used as input measurement signals for the method of the invention, the methane number can be calculated to essentially the same accuracy as when using a gas chromatograph. The deviation of the methane numbers is less than 2%.
When using the gas property data measured for charging purposes or when using supply network simulations, fuel gas customers can be informed at any time about current and possibly future fluctuations in the methane number without additional measurements. The gas transport network can also be controlled more flexibly without additional cost.
Further advantageous embodiments of the invention are characterised in the further and dependent claims.